1. Field of the Invention
The present invention relates to a motor system which includes a fuel cell serving as a power supply and more specifically, to a motor system in which torque of a drive motor is controlled when the fuel cell returns from an idle stop mode to an idle mode.
2. Description of Related Art
As an intermittent drive technique for fuel cells, for example, a technique described in [Patent Citation 1] below has been known. This [Patent Citation 1] describes a technique used when, in a fuel cell system including a fuel cell and a secondary battery supplying load power, intermittent operation of the fuel cell is performed in such a way that the fuel cell is stopped in a low load range while the secondary battery supplies load power. In this technique, a threshold for start and stop of the fuel cell is adjusted according to open circuit voltage (OCV) of the fuel cell which is stopped. Employment of such a technique can prevent fuel consumption for maintaining the open circuit voltage while the fuel cell is stopped and can also improve response of restart from the state with lowered open circuit voltage of the fuel cell which has stopped generating power.
[Patent Citation 1] Japanese Patent Laid-open Publication No. 2005-71797
In a motor such as a drive motor supplied with power from a fuel cell as described above, generally, since rotational speed of the drive motor increases slowly enough with respect to the response requirement for drive torque in a low rotational speed range (different depending on specifications of the drive motor), the change in drive motor power requirement per unit time is small. On the other hand, in a medium-high rotational speed range (different depending on the specifications of the drive motor), the drive motor can provide a maximum output, and the response requirement for the drive torque is substantially equal to the change in drive motor power requirement.
When such a drive motor is used to drive a fuel cell vehicle, the torque response requirement of the drive motor is set based on the perspective of driving performance of the vehicle, and the change in drive motor power requirement is generally large in the medium-high rotational speed range of the drive motor. Accordingly, the power generation of the fuel cell is basically a high-load and rapid action.
On the other hand, after remaining in idle stop (an idle stop mode) for a predetermined time, the fuel cell goes into an inactive state with an increased amount of condensed water, lowered temperature of the body of the fuel cell, an increased amount of non-fuel gas (nitrogen gas), and the like. If the fuel cell is shifted from the same state to the idle mode and performs high-load and rapid power generation, therefore, the fuel cell may deteriorate because of reduction in cell voltage. In order to avoid such deterioration and protect the fuel cell, it may be necessary to limit an output of the fuel cell.
Accordingly, when the idle stop is carried out in the medium-high rotational speed range of the drive motor, there is a lot of influence by the aforementioned output limitation. Power supplied from the fuel cell to the drive motor becomes insufficient, and the torque of the drive motor may be required to be limited. In the fuel cell vehicle with such a drive motor mounted thereon, there is a problem of deterioration in driving performance (acceleration performance) of the vehicle.
The present invention was therefore made in the light of the above description, and an object of the present invention is to provide a motor system in which torque of a drive motor supplied with power from a fuel cell is properly controlled.